It is known to place strings of pocketed or encased coil springs in a parallel fashion to create a mattress, inner spring unit or other body support foundation. A known method for making the strings of encased coil springs is to fold the encasing or pocketing material such that there is a crease on one longitudinal side and an opening on the opposite side. Compressed coils are inserted through the opening between the layers of pocketing material and the opening is scaled by a sealing system, such as a thermal sealing system or stitching. The layers of pocketing material are further sealed between each coil. The coils are then turned and expanded such that one end of the coil is facing the crease of the pocketing material and the other end is facing the sealed opening.
The number of coils in a string or row and the number of rows are dependent upon the coil spring diameter and the desired finished size of the mattress or the like. The construction of the mattress core may include a plurality of rows of parallel coils with the coils aligned in columns so that the coils line up in both longitudinal and lateral directions, or they may be nested in a honeycomb configuration wherein coils in one row are off-set from coils in the adjacent row.
It is known to connect the strings of coils in a coil to coil manner by applying an adhesive to the encased coils as illustrated in Stumpf U.S. Pat. Nos. 4,566,926 and 4,578,834 5,016,305 and 5,637,178, each of which are hereby incorporated by reference. Nested constructions where strings of coils are interlocked are illustrated in Stumpf U.S. Pat. No. 5,319,815 and German 4,040,220, each of which are hereby incorporated by reference. Other methods of connecting the strings of coils utilize metal clips known as hog rings or they may be stitched with twine which penetrates each string of coils.
Another method in the prior art is the use of strings of coils positioned in a frame with a web of nonwoven material sprayed on the top and bottom of the pocketed coil units, the spray nozzles being manually controlled. Alternatively, or in addition, to the spraying method, beads of hot melt adhesive may be dispensed onto the top and bottom surfaces of the strings of coils and a sheet of nonwoven material pressed against the adhesive containing surfaces.
In the prior art processes, control and distribution of the adhesive is difficult and inefficient. Some areas of the coil strings may receive too much adhesive while other areas may not receive a sufficient a mount of adhesive. Excess adhesive of course is economically inefficient, while risk of separation of the pocketed coil strings from the nonwoven material may result from too little adhesive. Moreover, when adhesive is spray ed there is a tendency for the spray nozzles to clog so that the flow of glue is obstructed. This results in a time consuming cleaning and maintenance program. Additionally, hot melt spraying requires the system to be heated about one hour before spraying can begin. Other difficulties presented by hot melt spraying and application of beads is that the hoses through which the hot melt flows must be insulated to maintain the temperature of the glue within the hose, thereby resulting in very heavy hoses. If the spraying process involves manually moving the spray nozzle to which the hose is attached to spray the adhesive, the heavy hoses result in the process being slow and fatiguing to the operators who often encounter minor burns from the glue and the nozzles.
A potential solution for the above-described disadvantages is disclosed in this inventor's pending U.S. patent application Ser. No. 09/024,536, filed Feb. 17, 1998, now U.S. Pat. No. 6.175,997 and hereby incorporated by reference in its entirety. In that method, the plurality of strings of pocketed coil springs are placed between top and bottom sheets of nonwoven webs of material, each web having a heat activated reinforcement netting. Thus, the strings are attached at their upper and lower surfaces to the sheets of material, rather than on their side surfaces to an adjacent string.
A problem that exists in all of these prior art processes is wear of the pocketing material at the points at which the coils in adjacent strings rub together. Over time, a hole is worn in the pocketing material, resulting in the metal coils rubbing together. The metal on metal rubbing creates undesirable noise when the mattress is in use. Breckel U.S. Pat. No. 4,907,309 provides a connecting wall made of elastic material between the strings of pocketed coil springs, with the wall being adhesively bonded to the strings. Although this reduces wear of the encasing material, an adhesive must still be manually applied to attach the strings to each other and the elastic material adds complexity and cost to the system and potentially reduces the firmness of the spring assembly.